Navigation Links
Actin


Actin is a globular protein that polymerize helicaly forming actin filaments (or microfilaments), which like the other two components of the cellular cytoskeleton form a three-dimensional network inside an eukariotic cell. Actin filaments provide mechanical support for the cell, determine the cell shape, enable cell movements (through pseudopods); and participate in certain cell junctions, in cytoplasmic streaming and in contraction of the cell during cytokinesis. In muscle cells they play an essential role, along with myosin, in muscle contraction. In the cytosol, actin is predominantly bound to ATP, but can also bind to ADP. An ATP-actin complex polymerizes faster and dissociates slower than an ADP-actin complex. Actin is also one of the most highly conserved proteins, differing by no more than 5% in species as diverse as algae and humans.

Contents

Microfilaments assembly

The globular Actin is known as G-actin, while the filamentous polymer composed of G-actin subunits (a microfilament), is called F-actin. The microfilaments are the thickest of the cytoskeleton, with only 7nm in diameter. Much like the microtubules, actin filaments are polar, with the plus (+) end elongating approximately 10 times faster than the minus (-) end. The process of actin polymerization, nucleation, starts with the association of three G-actin monomers into a trimer. ATP-actin then binds the plus (+) end, and the ATP is subsequently hydrolyzed, which reduces the binding strength between neighboring units and generally destabilizes the filament. ADP-actin dissociates from the minus end and the increase in ADP-actin stimulates the exchange of bound ADP for ATP, leading to more ATP-actin units. This rapid turnover is important for the cells movement.

The protein cofilin binds to ADP-actin units and promotes their dissociation from the minus end and prevents their reassembly. The protein profilin reverses this effect by stimulating the exchange of bound ADP for ATP. In addition, ATP-actin units bound to profilin will dissociate from cofilin and are then free to polymerize. Another important component in filament production is the Arp2/3 proteins, which serve as sites for nucleation, stimulating the formation of G-actin trimers. All of these three proteins are regulated by cell signaling mechanism.

Organization

Actin filaments are assembled in two general types of structures: bundles and networks. Actin-binding proteins dictate the formation of either structure since they cross-link actin filaments. Actin filaments have the appearance of a double-stranded helix.

Bundles

There are two types of actin bundles: parallel and contractile bundles. In parallel bundles, the filaments are spaced 14nm apart by the actin-bundling proteins fimbrin. Parallel bundles are responsible for the supporting a cells microvilli. In vertebrates, the actin-bundling protein villin is almost entirely found in the microvilli of intestinal cells.

Muscular Contraction

Main article: Muscle contraction

Together with myosin filaments actin it forms Actomyosin, which provides the mechanism for muscle contraction. Actin uses ATP for energy. The ATP allows, through hydrolysis, the myosin head to extend up and bind with the actin filament. The myosin head then releases after moving the actin filament in a relaxing or contracting movement by usage of ADP.

In contractile bundles, the actin-bundling protein actinin separates each filament by 40nm. This increase in distance allows the motor protein myosin to interact with the filament, enabling deformation or contraction. In the first case, one end of myosin is bound to the plasma membrane while the other end walks towards the plus end of the actin filament. This pulls the membrane into a different shape relative to the cell cortex. For contraction, the myosin molecule is usually bound to two separate filaments and both ends simultaneously walk towards their filament's plus end, sliding the actin filaments over each other. This results in the shorterning, or contraction, of the actin bundle (but not the filament). This mechanism is responsible for muscle contraction and cytokinesis, the division of one cell into two.

Networks

Actin networks, along with their actin-binding protein, filamin , form the cells cortex. This underlies the plasma membrane and is responsible for the shape of the cell.


'"/>


(Date:8/10/2015)... , August 10, 2015 The ... offers comprehensive analysis of the global border security ... generate revenues of $16.4bn in 2015. Now: Border ... is an example of the business critical issue you need ... read visiongain ,s objective analysis of how this will ...
(Date:8/6/2015)... TELTOW, Germany , August 6, 2015 ... SensoMotoric Instruments (SMI) shows the world,s first ... based on Epson,s Moverio BT-200 see-through head mounted ... this new solution, unprecedented quality and efficiency is brought ... context-sensitive displays. For the first time, professionals and researchers ...
(Date:8/5/2015)... 2015 According to a new ... Software, Services, Vertical (BFSI, Airport, IT & Telecom, Utilities, ... Forecast to 2020", published by MarketsandMarkets, defines and segments ... various segments. The global PIAM Market is estimated to ... by 2019, at a CAGR of 14.9 % from ...
Breaking Biology News(10 mins):Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 2Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 3Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 4Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 5Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 6Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 7Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 8Border Security Market Report 2015-2025 - Growth in Biometrics, ICT, Manned and Unmanned Vehicles, and Perimeter Surveillance 9SMI Shows First Eye Tracking Upgrade for Augmented Reality Glasses 2SMI Shows First Eye Tracking Upgrade for Augmented Reality Glasses 3Physical Identity and Access Management Market Worth $546.2 Million by 2019 2Physical Identity and Access Management Market Worth $546.2 Million by 2019 3Physical Identity and Access Management Market Worth $546.2 Million by 2019 4
... is available in German . The ... blocks inflammations. This was discovered by an international research team, ... as well as the doctoral candidate, Bjrn Spudy, are a ... the US and Great Britain were published yesterday, Sunday (7 ...
... November 8, 2010 The economic recession impacts many aspects ... This can have serious consequences for those suffering from chronic ... nutrition and menu planning play a key role in treatment, ... November/December issue of the Journal of Nutrition Education and ...
... Hamilton, ON (November 7, 2010) In an important ... human blood from adult human skin. The discovery, ... could mean that in the foreseeable future people needing blood ... like anemia will be able to have blood created from ...
Cached Biology News:Discovery of an anti-inflammatory substance 2Greater food insecurity from recession poses increased risk to low income individuals with diabetes 2
Other biology definitionOther Tags